Long-term photosynthetic acclimation to increased atmospheric CO(2) concentration in young birch (Betula pendula) trees

To study the long-term response of photosynthesis to elevated atmospheric CO(2) concentration in silver birch (Betula pendula Roth.), 18 trees were grown in the field in open-top chambers supplied with 350 or 700 &mgr;mol mol(-1) CO(2) for four consecutive growing seasons. Maximum photosynthetic rates, stomatal conductance and CO(2) response curves were measured over the fourth growing season with a portable photosynthesis system. The photosynthesis model developed by Farquhar et al. (1980) was fitted to the CO(2) response curves. Chlorophyll, soluble proteins, total nonstructural carbohydrates, nitrogen and Rubisco activity were determined monthly. Elevated CO(2) concentration stimulated photosynthesis by 33% on average over the fourth growing season. However, comparison of maximum photosynthetic rates at the same CO(2) concentration (350 or 700 &mgr;mol mol(-1)) revealed that the photosynthetic capacity of trees grown in an elevated CO(2) concentration was reduced. Analysis of the response curves showed that acclimation to elevated CO(2) concentration involved decreases in carboxylation efficiency and RuBP regeneration capacity. No clear evidence for a redistribution of nitrogen within the leaf was observed. Down-regulation of photosynthesis increased as the growing season progressed and appeared to be related to the source-sink balance of the trees. Analysis of the main leaf components revealed that the reduction in photosynthetic capacity was accompanied by an accumulation of starch in leaves (100%), which was probably responsible for the reduction in Rubisco activity (27%) and to a lesser extent for reductions in other photosynthetic components: chlorophyll (10%), soluble protein (9%), and N concentrations (12%) expressed on an area basis. Despite a 21% reduction in stomatal conductance in response to the elevated CO(2) treatment, stomatal limitation was significantly less in the elevated, than in the ambient, CO(2) treatment. Thus, after four growing seasons exposed to an elevated CO(2) concentration in the field, the trees maintained increased photosynthetic rates, although their photosynthetic capacity was reduced compared with trees grown in ambient CO(2).     

Effects of elevated atmospheric CO2 concentration on the nutrient uptake characteristics of Japanese larch (Larix kaempferi)

We evaluated the response of Japanese larch (Larix kaempferi Sieb. & Zucc.) to elevated atmospheric CO(2) concentration ([CO(2)]) (689 +/- 75 ppm in 2002 and 697 +/- 90 ppm in 2003) over 2 years in a field experiment with open-top chambers. Root activity was assessed as nitrogen, phosphorus and potassium uptake rates estimated from successive measurements of absorbed amounts. Dry matter production of whole plants was unaffected by elevated [CO(2)] in the first year of treatment, but increased significantly in response to elevated [CO(2)] in the second year. In contrast, elevated [CO(2)] increased the root to shoot ratio and fine root dry mass in the first year, but not in the second year. Elevated [CO(2)] had no effect on tissue N, P and K concentrations. Uptake rates of N, P and K correlated with whole-plant relative growth rates, but were unaffected by growth [CO(2)], as was ectomycorrhizal colonization, a factor assumed to be important for nutrient uptake in trees. We conclude that improved growth of Larix kaempferi in response to elevated [CO(2)] is accompanied by increased root biomass, but not by increased root activity.     

The influence of severe shoot pruning on growth,carbon and nitrogen status in young peach trees (Prunus persica)

One-year-old peach trees (Prunus persica (L.) Batsch) were severely pruned in July by removing 60% of the shoots. Tree responses were analyzed in terms of architecture and nutritional status. Tree growth was recorded from July to September by nondestructive (leaf production, thickening and branching of the remaining secondary axes) and destructive measurements (biomass partitioning and concentrations of total nitrogen (N) and nonstructural carbohydrates (NC) in specific tissues). The dry weights of pruned trees were lower than those of control trees at the end of the growing season (i.e., 2.5 months after pruning), whereas shoot:root ratios were restored to the initial values. Tree response occurred in two stages. During the first 24 days following pruning, the growth components of the remaining secondary axes were similar to the control, and new secondary axes were produced. During the next 17 days, increases in both diameter and branching of secondary axes contributed to the maintenance of pruned tree growth rate (similar to that of control trees) and restoration of initial shoot:root ratios. No significant effect of pruning was observed on NC concentrations, whereas N concentrations increased in several organs of the pruned trees during the first growth period. The transient increase in internal N availability contributed to the initiation of new axes and the restoration of a more functional biomass partitioning between shoots and roots.     

Growth and dry-matter partitioning of young Populus trichocarpa in response to carbon dioxide concentration and mineral nutrient availability

Young individuals of a single black cottonwood (Populus trichocarpa Torr. & Gray) clone were raised for three growing seasons in whole-tree chambers and exposed to either ambient or elevated atmospheric carbon dioxide concentration ([CO2]), with either a high or a low mineral nutrient supply, in a factorial experimental design. Nutrient availability had a larger effect on growth and dry matter partitioning than did [CO2]. Total biomass did not differ significantly with CO2 treatment when nutrient availability was low. However, elevated [CO2] increased whole-plant biomass by 47% in the high nutrient availability treatment. Carbon dioxide enrichment reduced leaf area ratio and specific leaf area significantly, but had no significant effect on mean leaf size or leaf mass ratio. Root mass ratio was significantly increased by elevated [CO2] at low, but not at high nutrient availability. A modified "demographic harvesting approach" made possible the retrospective estimation of stem and branch dry masses for different years. The relative growth rates of stem and branch were significantly enhanced by elevated [CO2] with high, but not with low nutrient availability. Canopy productivity index (CPI), i.e., the amount of stem and branch wood produced annually per unit leaf area, was raised 12% by elevated [CO2] when nutrient availability was high, but was reduced when nutrient availability was low, because of increased below ground allocation.     

Interaction of nutrient limitation and elevated CO2 concentration on carbon assimilation of a tropical tree seedling (Cedrela odorata)

Carbon assimilation by Cedrela odorata L. (Meliaceae) seedlings was investigated in ambient and elevated CO2 concentrations ([CO2]) for 119 days, using small fumigation chambers. A solution containing macro- and micronutrients was supplied at two rates. The 5% rate (high rate) was designed to avoid nutrient limitation and allow a maximum rate of growth. The 1% rate (low rate) allowed examination of the effect of the nutrient limitation-elevated CO2 interaction on carbon assimilation. Root growth was stimulated by 23% in elevated [CO2] at a high rate of nutrient supply, but this did not lead to a change in the root:shoot ratio. Total biomass did not change at either rate of nutrient supply, despite an increase in relative growth rate at the low nutrient supply rate. Net assimilation rates and relative growth rates were stimulated by the high rate of nutrient addition, irrespective of [CO2]. We used a biochemical model of photosynthesis to investigate assimilation at the leaf level. Maximum rate of electron transport (Jmax) and maximum velocity of carboxylation (Vcmax) did not differ significantly with CO2 treatment, but showed a substantial reduction at the low rate of nutrient supply. Across both CO2 treatments, mean Jmax for seedlings grown at a high rate of nutrient supply was 75 micromol m(-2) s(-1) and mean Vcmax was 27 micromol m(-2) s(-1). The corresponding mean values for seedlings grown at a low rate of nutrient supply were 36 micromol m(-2) s(-1) and 15 micromol m(-2) s(-1), respectively. Concentrations of leaf nitrogen, on a mass basis, were significantly decreased by the low nutrient supply rate, in proportion to the observed decrease in photosynthetic parameters. Chlorophyll and carbohydrate concentrations of leaves were unaffected by growth [CO2]. Because there was no net increase in growth in response to elevated [CO2], despite increased assimilation of carbon at the leaf level, we hypothesize that the rate of respiration of non-photosynthetic organs was increased.     

Genotypic variation in physiological and growth responses of Populus tremuloides to elevated atmospheric CO2 concentration

Physiological and biomass responses of six genotypes of Populus tremuloides Michx., grown in ambient t (357 micromol mol(-1)) or twice ambient (707 micromol mol(-1)) CO2 concentration ([CO2]) and in low-N or high-N soils, were studied in 1995 and 1996 in northern Lower Michigan, USA. There was a significant CO2 x genotype interaction in photosynthetic responses. Net CO2 assimilation (A) was significantly enhanced by elevated [CO2] for five genotypes in high-N soil and for four genotypes in low-N soil. Enhancement of A by elevated [CO2] ranged from 14 to 68%. Genotypes also differed in their biomass responses to elevated [CO2], but biomass responses were poorly correlated with A responses. There was a correlation between magnitude of A enhancement by elevated [CO2] and stomatal sensitivity to CO2. Genotypes with low stomatal sensitivity to CO2 had a significantly higher A at elevated [CO2] than at ambient [CO2], but elevated [CO2] did not affect the ratio of intercellular [CO2] to leaf surface [CO2]. Stomatal conductance and A of different genotypes responded differentially to recovery from drought stress. Photosynthetic quantum yield and light compensation point were unaffected by elevated [CO2]. We conclude that P. tremuloides genotypes will respond differentially to rising atmospheric [CO2], with the degree of response dependent on other abiotic factors, such as soil N and water availability. The observed genotypic variation in growth could result in altered genotypic representation within natural populations and could affect the composition and structure of plant communities in a higher [CO2] environment in the future.     

Drought and the diurnal patterns of stem CO2 efflux and xylem CO2 concentration in young oak (Quercus robur)

A young potted oak (Quercus robur L.) tree was subjected to drought by interrupting the water supply for 9 days. The tree was placed in a growth chamber in which daily patterns of temperature and radiation were constant. The effects of drought on the water and carbon status of the stem were examined by measuring stem sap flow rate, stem water potential, stem diameter variations, stem CO(2) efflux rate (F(CO2)) and xylem CO(2) concentration ([CO(2)*]). Before and after the drought treatment, diurnal fluctuations in F(CO2) and [CO(2)*] corresponded well with variations in stem temperature (T(st)). Daytime depressions in F(CO2) did not occur. During the drought treatment, F(CO2) still responded to stepwise changes in temperature, but diurnal fluctuations in F(CO2) were no longer correlated with diurnal fluctuations in T(st). From the moment daily growth rate of the stem became zero, diurnal fluctuations in F(CO2) became closely correlated with diameter variations, exhibiting clear daytime depressions. The depressions in F(CO2) were likely the result of a reduction in metabolic activity caused by the lowered daytime stem water status. Xylem [CO(2)*] showed clear daytime depressions in response to drought. When the tree was re-watered, F(CO2) and [CO(2)*] exhibited sharp increases, coinciding with an increase in stem diameter. After resumption of the water supply, daytime depressions in F(CO2) and [CO(2)*] disappeared and diurnal fluctuations in F(CO2) and [CO(2)*] corresponded again with variations in T(st).     

Long-term elevation of atmospheric CO(2) concentration and the carbon exchange rates of saplings of Pinus taeda L. and Liquidambar styraciflua L

The relationship between carbon exchange rate (CER) and internal CO(2) concentration was measured in leaves of saplings of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) grown from seed for more than 14 months at atmospheric CO(2) concentrations of either 350 or 500 microl l(-1). An elevated concentration of CO(2) during growth reduced CER at any given internal CO(2) concentration in sweetgum, but not in loblolly pine. Stomatal limitation of CER showed little response to concentration of CO(2) during measurement, but was higher in both species when grown at 500 than at 350 microl l(-1) CO(2). The net effect of a long-term increase in CO(2) concentration from 350 to 500 microl l(-1) was an increase in CER of loblolly pine, but a slight decrease in CER of sweetgum. It is suggested that the depression of CER in sweetgum resulted from a reduction in the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase.     

Kinetics of nitrogen uptake by Populus tremuloides in relation to atmospheric CO(2) and soil nitrogen availability

Sustained increases in plant production in response to elevated atmospheric carbon dioxide (CO(2)) concentration may be constrained by the availability of soil nitrogen (N). However, it is possible that plants will respond to N limitation at elevated CO(2) concentration by increasing the specific N uptake capacity of their roots. To explore this possibility, we examined the kinetics of (15)NH(4) (+) and (15)NO(3) (-) uptake by excised roots of Populus tremuloides Michx. grown in ambient and twice-ambient CO(2) concentrations, and in soils of low- and high-N availability. Elevated CO(2) concentration had no effect on either NH(4) (+) or NO(3) (-) uptake, whereas high-N availability decreased the capacity of roots to take up both NH(4) (+) and NO(3) (-). The maximal rate of NH(4) (+) uptake decreased from 12 to 8 &mgr;mol g(-1) h(-1), and K(m) increased from 49 to 162 &mgr;mol l(-1), from low to high soil N availability.Because NO(3) (-) uptake exhibited mixedkinetics over the concentration range we used (10-500 &mgr;mol l( -1)), it was not possible to calculate V(max) and K(m). Instead, we used an uptake rate of 100 &mgr;mol g(-1) h(-1) as our metric of NO(3) (-) uptake capacity, which averaged 0.45 and 0.23 &mgr;mol g(-1) h(-1) at low- and high-N availability, respectively. The proximal mechanisms for decreased N uptake capacity at high-N availability appeared to be an increase in fine-root carbohydrate status and a decrease in fine-root N concentration. Both NH(4) (+) and NO(3) (-) uptake were inversely related to fine-root N concentration, and positively related to fine-root total nonstructural carbohydrate concentration. We conclude that soil N availability, through its effects on fine-root N and carbohydrate status, has a much greater influence on the specific uptake capacity of P. tremuloides fine roots than elevated atmospheric CO(2). In elevated atmospheric CO(2), changes in N acquisition by P. tremuloides appeared to be driven by changes in root architecture and biomass, rather than by changes in the amount or activity of N-uptake enzymes.     

Ammonium and nitrate acquisition by plants in response to elevated CO2 concentration: the roles of root physiology and architecture

We examined changes in root system architecture and physiology and whole-plant patterns of nitrate reductase (NR) activity in response to atmospheric CO2 enrichment and N source to determine how changes in the form of N supplied to plants interact with rising CO2 concentration ([CO2]). Seedlings of Betula alleghaniensis Britt. and Pinus strobus L., which differ in growth rate, root architecture, and the partitioning of NR activity between leaves (Betula) and roots (Pinus), were grown in ambient (400 microl l(-1)) and elevated (800 microl l(-1)) [CO2] and supplied with either nitrate (NO3-) or ammonium (NH4+) as their sole N source. After 15 weeks of growth, plants were harvested and root system architecture, N uptake kinetics, and NR activity measured. Betula alleghaniensis responded to elevated [CO2] with significant increases in growth, regardless of the source of N. Pinus strobus showed no significant response in biomass production or allocation to elevated [CO2]. Both species exhibited significantly greater growth with NH4+ than with NO3-, along with lower root:shoot biomass ratios. Betula showed significant increases in total root length in response to elevated [CO2]. However, root N uptake rates in Betula (for both NO3- and NH4+) were either reduced or unchanged by elevated [CO2]. Pinus showed the opposite response to elevated [CO2], with no change in root architecture, but an increase in maximal uptake rates in response to elevated [CO2]. Nitrate reductase activity (on a mass basis) was reduced in leaves of Betula in elevated [CO2], but did not change in other tissues. Nitrate reductase activity was unaffected by elevated [CO2] in Pinus. Scaling this response to the whole-plant, NR activity was reduced in elevated [CO2] in Betula but not in Pinus. However, because Betula plants were larger in elevated [CO2], total whole-plant NR activity was unaffected.     

Gas exchange and dry matter allocation responses to elevation of atmospheric CO(2) concentration in seedlings of three tree species

Photosynthetic rates of 13-month-old Pinus radiata D. Don, Nothofagus fusca (Hook f.) ?rst. and Pseudotsuga menziesii (Mirb.) Franco seedlings grown and measured at elevated atmospheric concentrations of CO(2) (~620 microl l(-1)) were 32 to 55% greater than those of seedlings grown and measured at ambient (~310 microl l(-1)) concentrations of CO(2). Seedlings grown in ambient and elevated concentrations of CO(2) had similar rates of photosynthesis when measured at ~620 microl l(-1) CO(2), but when measured at ~310 microl l(-1) CO(2), the P. radiata and N. fusca seedlings which were grown at elevated CO(2) had lower rates of photosynthesis than the seedlings grown at an ambient concentration of CO(2). Stomatal conductances in general were lower when measured at ~620 microl l(-1) CO(2) than at ~310 microl l(-1) CO(2). Stomatal conductances declined in all species grown at both CO(2) concentrations when the leaf-air water vapor concentration gradient (DeltaW) was increased from 10 to 20 mmol H(2)O mol(-1) air. The percent enhancement in photosynthesis for P. radiata and P. menziesii at elevated CO(2) was greater at 20 mmol than at 10 mmol DeltaW, suggesting that elevated CO(2) may moderate the effects of atmospheric water stress. Dry matter allocation patterns were not significantly different for plants grown in ambient or high CO(2) air.     

Isotopic carbon discrimination and leaf nitrogen content of Erica arborea L. along a CO(2) concentration gradient in a CO(2) spring in Italy

We studied a Mediterranean species (Erica arborea L.) growing in a CO(2) spring in Italy that was naturally exposed for generations to a gradient of atmospheric CO(2) concentrations. The CO(2) concentration gradient to which different individual plants were exposed was determined by an indirect method based on radioisotope analysis. The stable carbon isotope ratio of sampled leaves was determined by mass spectrometry, and isotopic discrimination was then calculated. Leaf nitrogen, specific leaf area, total soil nitrogen, soil organic matter content and soil pH were also measured. In one group of plants, grown on a homogeneous soil and exposed to moderate CO(2) enrichment, isotopic discrimination was significantly reduced in response to increasing CO(2) concentrations, whereas the intercellular CO(2) concentration and leaf nitrogen content were almost unaffected. In a second group of plants, grown along a gradient of CO(2) concentration and soil nitrogen content, leaf nitrogen content was reduced when nitrogen availability was limiting. However, when soil nitrogen was available in excess, even very high CO(2) concentrations did not result in increased discrimination or reduced leaf nitrogen content in the long term. The results are discussed with respect to current theories about the long-term CO(2) response of plants based on several years of experimentation with elevated atmospheric CO(2) concentrations under controlled conditions.     

Relationship between photosynthesis and leaf nitrogen concentration in ambient and elevated [CO2] in white birch seedlings

To study the effects of elevated CO2 concentration ([CO2]) on relationships between nitrogen (N) nutrition and foliar gas exchange parameters, white birch (Betula papyrifera Marsh.) seedlings were exposed to one of five N-supply regimes (10, 80, 150, 220, 290 mg N l(-1)) in either ambient [CO2] (360 micromol mol(-1)) or elevated [CO2] (720 micromol mol(-1)) in environment-controlled greenhouses. Foliar gas exchange and chlorophyll fluorescence were measured after 60 and 80 days of treatment. Photosynthesis showed a substantial down-regulation (up to 57%) in response to elevated [CO2] and the magnitude of the down-regulation generally decreased exponentially with increasing leaf N concentration. When measured at the growth [CO2], elevated [CO2] increased the overall rate of photosynthesis (P(n)) and instantaneous water-use efficiency (IWUE) by up to 69 and 236%, respectively, but decreased transpiration (E) and stomatal conductance (g(s)) in all N treatments. However, the degree of stimulation of photosynthesis by elevated [CO2] decreased as photosynthetic down-regulation increased from 60 days to 80 days of treatment. Elevated [CO2] significantly increased total photosynthetic electron transport in all N treatments at 60 days of treatment, but the effect was insignificant after 80 days of treatment. Both P(n) and IWUE generally increased with increasing leaf N concentration except at very high leaf N concentrations, where both P(n) and IWUE declined. The relationships of P(n) and IWUE with leaf N concentration were modeled with both a linear regression and a second-order polynomial function. Elevated [CO2] significantly and substantially increased the slope of the linear regression for IWUE, but had no significant effect on the slope for P(n). The optimal leaf N concentration for P(n) and IWUE derived from the polynomial function did not differ between the CO2 treatments when leaf N was expressed on a leaf area basis. However, the mass-based optimal leaf N concentration for P(n) was much lower in seedlings in elevated [CO2] than in ambient [CO2] (31.88 versus 37.00 mg g(-1)). Elevated [CO2] generally decreased mass-based leaf N concentration but had no significant effect on area-based leaf N concentration; however, maximum N concentration per unit leaf area was greater in elevated [CO2] than in ambient [CO2] (1.913 versus 1.547 g N m(-2)).     

Interactive effects of elevated CO2 concentration and nitrogen supply on partitioning of newly fixed 13C and 15N between shoot and roots of pedunculate oak seedlings (Quercus robur)

Pedunculate oak (Quercus robur L.) seedlings were grown for 3 or 4 months (second- and third-flush stages) in greenhouses at two atmospheric CO2 concentrations ([CO2]) (350 or 700 micromol mol(-1)) and two nitrogen fertilization regimes (6.1 or 0.61 mmol N l(-1) nutrient solution). Combined effects of [CO2] and nitrogen fertilization on partitioning of newly acquired carbon (C) and nitrogen (N) were assessed by dual 13C and 15N short-term labeling of seedlings at the second- or third-flush stage of development. In the low-N treatment, root growth, but not shoot growth, was stimulated by elevated [CO2], with the result that shoot/root biomass ratio declined. At the second-flush stage, overall seedling biomass growth was increased (13%) by elevated [CO2] regardless of N fertilization. At the third-flush stage, elevated [CO2] increased growth sharply (139%) in the high-N but not the low-N treatment. Root/shoot biomass ratios were threefold higher in the low-N treatment relative to the high-N treatment. At the second-flush stage, leaf area was 45-51% greater in the high-N treatment than in the low-N treatment. At the-third flush stage, there was a positive interaction between the effects of N fertilization and [CO2] on leaf area, which was 93% greater in the high-N/elevated [CO2] treatment than in the low-N/ambient [CO2] treatment. Specific leaf area was reduced (17-25%) by elevated [CO2], whereas C and N concentrations of seedlings increased significantly in response to either elevated [CO2] or high-N fertilization. At the third-flush stage, acquisition of C and N per unit dry mass of leaf and fine root was 51 and 77% greater, respectively, in the elevated [CO2]/high-N fertilization treatment than in the ambient [CO2]/low-N fertilization treatment. However, there was dilution of leaf N in response to elevated [CO2]. Partitioning of newly acquired C and N between shoot and roots was altered by N fertilization but not [CO2]. More newly acquired C and N were partitioned to roots in the low-N treatment than in the high-N treatment.     

Resistance against oxidative stress in leaves of young beech trees grown in model ecosystems with different soil qualities, elevated CO2, and lachnid infestation

Summary The aim of the present study was to investigate whether the resistance of beech foliage(Fagus sylvatica) against oxidative stress was affected by soil quality, nitrogen or CO₂ fertilisation, or lachnid infestation(Phyllaphis fagi). For this purpose young beech trees were grown for four years in reconstructed calcareous or acidic forest soils in open top chambers under ambient or elevated CO₂ concentrations with two levels of nitrogen fertilisation. At harvest lachnid colonisation was observed, preferentially on leaves from trees in calcareous soil and on leaves from trees fertilised with the high nitrogen level. General leaf characteristics such as pigment concentrations, dry mass, and leaf mass ber area were not affected by the soil type, nitrogen fertilisation or CO₂ regime. Leaves colonised with lachnids displayed slightly increased leaf mass per area. When the stress resistance was challenged by exposure to paraquat — a herbicide inducing oxidative stress — leaves from trees grown on calcareous soil maintained significantly longer membrane integrity and, thus, were better protected against stress than leaves from trees on acidic soil. Other experimental variables had negligible or no effects on the resistance against oxidative stress.      

Effects of soil temperature and elevated atmospheric CO2 concentration on gas exchange, in vivo carboxylation and chlorophyll fluorescence in jack pine and white birch seedlings

One-year-old jack pine (Pinus banksiana Lamb.) and current-year white birch (Betula papyrifera Marsh.) seedlings were grown in ambient (360 ppm) or twice ambient (720 ppm) atmospheric CO2 concentration ([CO2]) and at three soil temperatures (Tsoil = 7, 17 and 27 degrees C initially, increased to 10, 20 and 30 degrees C two months later, respectively) in a greenhouse for 4 months. In situ foliar gas exchange, in vivo carboxylation characteristics and chlorophyll fluorescence were measured after 2.5 and 4 months of treatment. Low Tsoil suppressed net photosynthetic rate (Pn), stomatal conductance (g(s)) and transpiration rate (E) in jack pine in both CO2 treatments and g(s) and E in white birch in ambient [CO2], but enhanced instantaneous water-use efficiency (IWUE) in both species after 2.5 months of treatment. Treatment effects on g(s) and E remained significant throughout the 4-month study. Low Tsoil reduced maximal carboxylation rate (Vcmax) and PAR-saturated electron transport rate (Jmax) in jack pine in elevated [CO2] after 2.5 months of treatment, but not after 4 months of treatment. Low Tsoil increased actual photochemical efficiency of photosystem II (PSII) in the light (DeltaF/Fm') in jack pine, but decreased DeltaF/Fm' in white birch after 4 months of treatment. In response to low Tsoil, photosynthetic linear electron transport to carboxylation (Jc) decreased in jack pine after 2.5 months and in white birch after 4 months of treatment. Low Tsoil increased the ratio of the photosynthetic linear electron transport to oxygenation (Jo) to the total photosynthetic linear electron transport rate through PSII (Jo/J(T)) in both species after 2.5 months of treatment, but the effects became statistically insignificant in white birch after 4 months of treatment. High Tsoil decreased foliar N concentration in white birch. Elevated [CO2] increased Pn, IWUE and Jc but decreased Jo/J(T) in both species at both measurement times except Jc in white birch after 2.5 months of treatment. Elevated [CO2] also decreased g(s) and E in white birch at high Tsoil, Vcmax in both species and triose phosphate utilization in white birch at low Tsoil after 4 months of treatment, and DeltaF/Fm' in white birch after 2.5 months of treatment. Elevated [CO2] also increased foliar N concentration in both species. Low Tsoil caused no permanent damage to PSII in either species, but jack pine responded and acclimated to low Tsoil more quickly than white birch. Photosynthetic down-regulation and a decrease in photosynthetic electron transport to photorespiration occurred in both species in response to elevated [CO2].     

Growth and carbon accumulation in root systems of Pinus taeda and Pinus ponderosa seedlings as affected by varying CO(2), temperature and nitrogen

It has been hypothesized that increasing atmospheric CO(2) concentration enhances accumulation of carbon in fine roots, thereby altering soil carbon dynamics and nutrient cycling. To evaluate possible changes to belowground pools of carbon and nitrogen in response to elevated CO(2), an early and a late successional species of pine (Pinus taeda L. and Pinus ponderosa Dougl. ex Laws, respectively) were grown from seed for 160 days in a 35 or 70 Pa CO(2) partial pressure at low or high temperature (30-year weekly mean and 30-year weekly mean + 5 degrees C) and a soil solution nitrogen concentration of 1 or 5 mM NH(4)NO(3) at the Duke University Phytotron. Seedlings were harvested at monthly intervals and growth parameters of the primary root, secondary root and tap root fractions evaluated. Total root biomass of P. ponderosa showed a positive CO(2) response (105% increase) (P = 0.0001) as a result of significant increases in all root fractions in the elevated CO(2) treatment, but all other main effects and interactions were insignificant. In P. taeda, there were significant interactions between CO(2) and temperature (P = 0.04) and CO(2) and nitrogen (P = 0.04) for total root biomass. An allometric analysis indicated that modulation of the secondary root fraction was the main response of the trees to altered environmental conditions. In P. ponderosa, there was an increase in the secondary root fraction relative to the primary and tap root fractions under conditions of low temperature. In P. taeda, there was a shift in carbon accumulation to the secondary roots relative to the primary roots under low temperature and low nitrogen. Neither species exhibited shifts in carbon accumulation in response to elevated CO(2). We conclude that both species have the potential to increase belowground biomass substantially in response to rising atmospheric CO(2) concentration, and this response is sensitive to temperature and nitrogen in P. taeda. Both species displayed small shifts in belowground carbon accumulation in response to altered temperature and nitrogen that may have substantial ecosystem consequences over time.     

Genetic variation in foliar nutrient concentration in relation to foliar carbon isotope composition and tree growth with clones of the F1 hybrid between slash pine and Caribbean pine

The objectives of this study were: (1) to quantify the genetic variation in foliar nutrient concentration in relation to foliar carbon isotope composition (δ¹³C) and tree growth of 122 clones of ca. 4-year-old F₁ hybrids between slash pine (Pinus elliottii Engelm var. elliottii) and Caribbean pine (P. caribaea var. hondurensis Barr. et Golf.) grown at two experimental sites with different water and nutrient availability in southeast Queensland, Australia and (2) to examine the potential of using foliar nutrient concentration of the 4-year-old tree canopies for selecting elite F₁ hybrid pine clones with improved nutrient-use efficiency (NUE) and water-use efficiency (WUE), and ultimately enhanced tree growth under ambient growing conditions. There were significant differences in foliar nutrient concentrations between two canopy positions (upper outer and lower outer canopy) sampled, between summer and winter, and between the two sites. This highlights that foliar nutrient concentrations are influenced by sampling and environment. Significant genetic variations in foliar nutrient concentrations were detected between the clones, between the female parents, and between the male parents of the clones in both sampling seasons at both sites. Depending on the nutrient concerned, canopy position, season, and site sampled, the clones accounted for 4.7–33.9% of the total variation in foliar nutrient concentrations, the clone female parents for 0–25.1% and the clone male parents for 0–28.6%. The site-by-clone interactions were statistically significant for foliar N, P, Mg, Cu, Zn, Mn, Fe and mineral concentrations at the upper outer canopy in summer, and for foliar N concentration in winter. There were significant, positive correlations between clone means of foliar δ¹³C and N concentration at the upper outer canopy in summer for the wet site, while clone foliar δ¹³C was also positively related to clone foliar N concentration at both canopy positions in summer for the dry site. This suggests that clone WUE as reflected in foliar δ¹³C may be improved by selecting elite clones with higher foliar N concentration and increased photosynthesis, leading to enhanced tree growth when both water and N are the major growth-limiting factors. This is supported by the positive correlation detected between clone tree height and foliar N concentration at the upper outer canopy for both sites. Thus, foliar nutrient (particularly N) concentration, together with foliar δ¹³C, may be useful for assisting in selection of exotic pine clones with improved NUE and WUE, and enhanced tree growth under the nutrient- and water-limiting environments.     

Changes in root growth and relationships between plant organs and root hydraulic traits in American elm (Ulmus americana L.) and red oak (Quercus rubra L.) seedlings due to elevated CO2 level

The relationships between plant organs and root hydrological traits are not well known and the question arises whether elevated CO2 changes these relationships. This study attempted to answer this question. A pseudo-replicated experiment was conducted with two times 24 American elm (Ulmus americana L.) and 23 and 24 red oak (Quercus rubra L.) seedlings growing in ambient CO2 (around 360 μmol·L–1) and 540 ± 7.95 μmol·L–1 CO₂ in a greenhouse. After 71 days of treatment for American elm and 77 days for red oak, 14 American elm and 12 red oak seedlings from each of the two CO2 levels were randomly selected in order to examine the flow rate of root xylem sap, root hydraulic conductance, total root hydraulic conductivity, fine root and coarse root hydraulic conductivity. All seedlings were harvested to investigate total plant biomass, stem biomass and leaf biomass, leaf area, height, basal diameter, total root biomass, coarse root biomass and fine root biomass. The following conclusions are reached: 1) plant organs respond to the elevated CO2 level earlier than hydraulic traits of roots and may gradually lead to changes in hydraulic traits; 2) plant organs have different relationships with hydraulic traits of roots and elevated CO2 changes these relationships; the changes may be of importance for plants as means to acclimatize to changing environments; 3) biomass of coarse roots increased rather more than that of fine roots; 4) Lorentzian and Caussian models are better in estimating the biomass of seedlings than single-variable models.